The Sagnac interferometer gyroscope (sig) is a well known device for measuring rate of inertial rotation. The output beam from a laser is split and one portion is passed through an open ended fiber optic loop in a first direction, for example, clockwise, while the second portion is passed through the loop counterclockwise. One portion is passed through a phase modulator prior to coupling to the optical fiber loop, while the light traveling in the opposite direction is coupled through the modulator after passing through the fiber optic loop. The phase difference .DELTA..phi. between the two outputs is representative of the inertial rate of rotation of the loop, .OMEGA., according to the equation, ##EQU1## where:
N is the number of the loops in the fiber optic ring,
A is the area enclosed by the light path,
C is the speed of light in a vacuum,
.lambda..sub.o is the vacuum wavelength of the source light
The sensitivity, .epsilon., of the gyroscope is expressed as ##EQU2##
Where I.sub.o is the maximum interferometer output intensity
As indicated from equations (1) and (2), as the phase angle approaches zero, so does the sensitivity. In order to maximize the signal, it is conventional in Sagnac interferometers to have a 90.degree. phase bias and employ a switching technique so that measurements are made at points of maximum sensitivity, namely (-.pi./2).sub.o and (+.pi./20.sub.o).
One of the problems associated with interferometers of this type has been false indications of rate of inertial rotation because of variations in amplifier gain or light source intensity. Either one of these variations would appear as a change in inertial rotation.
A Description of a Sagnac interferometer gyroscope with some discussion of these sources of error is presented in the following papers: J. L. Davis and S. Ezekiel, "Techniques for Shot-Noise-Limited Inertial Rotation Measurement Using A Multiturn Fiber Sagnac Interferometer", SPIE Vol. 157 Laser Inertial Rotation Sensors (1978) pp. 131-136;
J. M. Marin and J. T. Winkler, "Fiber Optic Laser Gyro Signal Detection and Processing Technique", SPIE Vol. 139 Guided Wve Optical Systems and Devices (1978), pp. 98-102;